The aim of this proposed research is to examine pathways related to tranfer of cholesterol in plasma and pathways of removal of cholesterol from the plasma compartment. Experiments will be carried out to elucidate the following metabolic pathways: 1. Generation of free cholesterol during lipolysis of the triglyceride-rich lipoproteins, chylomicrons and VLDL and its transfer to HDL and to cell membranes; 2. Esterification of the cholesterol in HDL by the LCAT reaction, and the effects of accumulated cholesterol ester on the composition and structure of HDL and HDL subpopulations; 3. Transfer of cholesterol ester from HDL to other lipoproteins by the activity of the cholesterol ester transfer protein and the effects of this process on the composition and structure of the cholesterol ester depleted HDL and the cholesterol ester acceptor lipoproteins; 4. Exit of cholesterol and cholesterol ester from the plasma compartment with irreversible catabolism of lipoproteins (remnants, LDL and HDL) and in molecular form independent of lipoprotein catabolism. Enzymes transfer proteins and interactions of lipoprotein particles with one another and with cells regulate cholesterol accumulation in lipoproteins and cholesterol transfer to cells. To study these complex processes, a proper methodology for isolation and characterization of lipoproteins and for purification of enzymes and transfer proteins have been established. Experimental procedures will include in vitro incubation systems, studies in isolated perfused rat organs (including the functionally hepatectomized rat) and in vivo studies in rats, an animal devoid of cholesterol ester transfer activity. These experiments will quantitatively evaluate the overall process of plasma cholesterol transport and will determine the effects of this process on the composition and structure of lipoproteins, in particular HDL. Cholesterol transport in some lipoproteins (VLDL, LDL) is responsible for initiation of atherosclerosis; HDL cholesterol in contrast is associated with prevention of atherosclerosis. The study therefore is relevant to health problems related to atherosclerotic diseases.